A number of industrial manufacturing processes require the use of high-temperature, corrosion-resistant electrodes. Examples include the Hall-Heroult process for producing aluminum from alumina-bearing ores, and electric arc furnaces for the manufacture of steel and for the melting of refractory metals and ceramics. Industrial processes for energy generation from fossil fuels also require the use of electrode materials that are high-melting and resistant to degradation. Examples of the latter include energy production by the magnetohydrodynamic (MHD) process and solid oxide fuel cells.
It is known to produce aluminum by molten salt electrolysis of aluminum oxide dissolved in a bath of aluminum sodium fluoride (AlF.sub.3.3NaF) or so-called cryolite, by using a carbon anode. This electrolysis is usually conducted at about 900.degree.-1000.degree. C.
When aluminum is produced by using a carbon anode, the carbon anode is oxidized and consumed by about 330 kg theoretically and about 400-450 kg actually per ton of aluminum due to oxygen produced through the decomposition of aluminum oxide. For this reason, it is necessary to continuously adjust the position of the electrode to maintain it at a constant level, and it is also required to replace the anode by a new one before it is completely consumed. These are economical and operational defects.
In the electrolytic production of aluminum by the Hall-Heroult process a cryolite melt with Al.sub.2 O.sub.3 dissolved in it is electrolyzed at 940.degree.-1000.degree. C. The aluminum which separates out in the process collects on the cathodic carbon floor of the electrolysis cell whilst CO.sub.2 and to a small extent CO are formed at the carbon anode. The anode is thereby burnt away.
For the reaction EQU Al.sub.2 O.sub.3+ 3/2C.fwdarw.2Al+3/2CO.sub.2
this combustion should in theory consume 0.334 kg C/kg Al; in practice however, up to 0.5 kg C/kg Al is consumed.
The burning away of the anodes has a number of disadvantages. In order to obtain aluminum of acceptable purity, a relatively pure coke with low ash content has to be used to produce the anode carbon. The pre-baked carbon anodes have to be advanced from time to time in order to maintain the optimum inter-polar distance between the anode surface and the surface of the aluminum. Periodically the pre-baked anodes when consumed have to be replaced by new ones. Soderberg anodes have to be repeatedly charged with new material. In the case of pre-baked anodes a separate manufacturing plant typically is necessary.
Accordingly, the manufacture of carbon anodes and their use in aluminum production is laborious and expensive.
The direct decomposition of Al.sub.2 O.sub.3 to its elements: EQU Al.sub.2 O.sub.3 .fwdarw.2Al+3/2O.sub.2
using an anode where no reaction with the oxygen takes place is therefore of greater interest. With non-reactive anodes, oxygen, which can be re-used industrially, is released, and the above mentioned disadvantages of the carbon anodes also disappear. This anode is particularly favorable for a sealed furnace the waste gases of which can be easily collected and purified. Accordingly, in order to reduce greenhouse gas emissions and to allow for lower energy costs, manufacturers of aluminum have long sought inert anode materials to replace carbon in the Hall-Heroult Cell for aluminum production.
There are many concomitant requirements and considerations that must be satisfied in or to produce such replacement material:
1. It must be thermally stable up to 1000.degree. C. PA0 2. The specific electrical resistivity must be very small so that the voltage drop in the anode is a minimum. At 1000.degree. C. the specific resistivity should be comparable with, or smaller than that of anode carbon. The specific resistivity should also be as independent of temperature as possible so that the voltage drop in the anode remains as constant as possible even when temperature changes occur in the bath. PA0 3. Oxidizing gases are formed on the anode therefore the anodes must be resistant to oxidation. PA0 4. The anode material should be insoluble in a fluoride or oxide melt. PA0 5. The anode should have adequate resistance to damage from temperature change so that on introduction into the molten charge or when temperature changes occur during electrolysis it is not damaged. PA0 6. Anode corrosion should be negligibly small. If nevertheless some kind of anode product should enter the bath then neither the electrolyte, the separated metal nor the power output should be affected. PA0 7. On putting the anodes into service in the industrial production of aluminum, they must be stable when in contact with the liquid electrolyte, have no influence on the purity of the aluminum obtained, and operate economically. Obviously the number of materials which even approach fulfilling these extremely severe criteria is very limited. PA0 8. The anode should have adequate mechanical strength.
In applications directed toward the electrowinning of metals such as aluminum or similar electrolysis reactions conducted at high temperature an inert anode thus must first be resistant to dissolution by cryolite-based melts. It must also be electrically conductive and mechanically robust. The replacement material must likewise be resistant to reduction by molten species, such as molten aluminum.
As an approach to obviate the above-mentioned defects in the carbon electrode, various non-consumable anodes have been developed. For example, a method using an oxygen ion-conductive anode consisting mainly of zirconium oxide has been proposed (British Patent Specification No. 1,152,124). This method, however, is disadvantageous in that it requires an apparatus for removing oxygen produced and the operation is complex. A method using an anode consisting of electronic conductive metal oxide containing at least 80% by weight of tin oxide has also been proposed (British Pat. Specification No. 1,295,117). This method is also disadvantageous in that the anode has poor chemical resistance to the molten salt.
In the Swiss Pat. No. 520 779 an anode made of ceramic oxide material in particular 80-99% SnO.sub.2 is described. However this anode was shown to be problematic in that it showed a certain amount of loss and as a result of this the aluminum obtained amongst other things was made impure by the inclusion of tin which in most cases is undesirable.
As an improvement, U.S. Pat. No. 3,960,678 to Alder disclosed a process for operating a cell for the electrolysis of aluminum oxide with one or more anodes, the working surface of which is of ceramic oxide material. However, according to the patent, the process requires a current density above a minimum value to be maintained over the whole anode surface which comes in contact with the molten electrolyte to minimize the corrosion of the anode. This patent discloses SnO.sub.2, Fe.sub.2 O.sub.3, Fe.sub.3 O.sub.4, Cr.sub.2 O.sub.3, Co.sub.3 O.sub.4, NiO or ZnO as base materials. Without additives, SnO.sub.2 cannot be made into a densely sintered product and it exhibits a relatively high specific resistivity at 1000.degree. C. Additions of other oxides in a concentration of 0.01-20%, preferably 0.05-2% have to be made in order to improve such properties of pure tin oxide. To improve the sinterability, the compactness and the conductivity of the SnO.sub.2, Alder teaches additions of one or more of the oxides of the following metals are found to be useful: Fe, Cu, Mn, Nb, Zn, Co, Cr, W, Sb, Cd, Zr, Ta, In, Ni, Ca, Ba, Bi.
Numerous efforts have been made to provide an inert electrode having the above characteristics but apparently without the required degree of success to make it economically feasible. That is, the inert electrodes in the art appear to be reactive to an extent which results in contamination of the metal being produced as well as consumption of the electrode. For example, U.S. Pat. No. 4,039,401 reports that extensive investigations were made to find non-consumable electrodes for molten salt electrolysis of aluminum oxide, and that spinel structure oxides or perovskite structure oxides have excellent electronic conductivity at a temperature of 900.degree. to 1000.degree. C., exhibit catalytic action for generation of oxygen and exhibit chemical resistance. However, commercial use of these anodes has not been realized because they are not sufficiently inert.
Thus, it can be seen that there remains a great need for an electrode which is substantially inert or is resistant to attack by molten salts or molten metal to avoid contamination and its attendant problems.
It has been proposed that an inert electrode be constructed using ceramic oxide compositions having a metal powder dispersed therein for the purpose of increasing the conductivity of the electrode. For example, when an electrode composition is formulated from NiO and Fe.sub.2 O.sub.3, a highly suitable metal for dispersing through the composition is nickel which may increase the conductivity of the electrode by as much as 30 times.
However, it has been found that the search for inert electrode materials possessing the requisite chemical inertness and electrical conductivity is further complicated by the need to preserve certain mechanical characteristics which may be either enhanced or impaired by modifications to enhance the chemical resistance or electrical conductivity. For example, the electrode should possess certain minimum mechanical strength characteristics tested by the modulus of rupture, fracture toughness and expansion and resistance to thermal shock of the electrode material as well as the ability to weld electrical connections thereto must also be taken into account. An article entitled "Displacement Reactions in the Solid State" by R. A. Rapp et al, published May 1973, in Volume 4 of Metallurgical Transactions, at pages 1283-1292, points out the different morphologies which can result from the addition of a metal or metal alloy to an oxide mixture. The authors show that some additions result in layers of metal or metal oxides while others form aggregate arrangements which may be lamellar or completely interwoven. The authors suggest that interwoven-type microstructures should be ideal for the transfer of stresses and resistance to crack propagation and demonstrated that such were not fractured by rapid cooling. The authors suggested that such an interwoven structure would be useful in the preparation of porous electrodes for fuel cells or as catalysts for reactions between gases by selective dissolution of either the metal or oxide phase.
U.S. Pat. No. 4,039,401 discloses a non-consumable electrode for electrolytic production of aluminum containing at least 50% by weight of spinel structure oxide having the general formula XYY'O.sub.4 (wherein X is a divalent or tetravalent metal, Y and Y' may be either the same or different and are trivalent or divalent metals, O is oxygen atom, provided that when X is a divalent metal, Y and Y' are selected from trivalent metals but the spinel structure oxides are excluded in which both Y and Y' are trivalent iron, Fe(III), and when X is a tetravalent metal, Y and Y' are selected from divalent metals), or a perovskite structure oxide having the general formula RMO.sub.3 (wherein R is a monovalent, divalent or trivalent metal, M is a pentavalent, tetravalent or trivalent metal, O is oxygen atom, provided that when R is a monovalent metal, M is selected from pentavalent metals, and when R is a divalent metal, M is selected from tetravalent metals, and when R is a trivalent metal, M is selected from trivalent metals), or a mixture thereof, said oxides exhibiting chemical durability against the molten salt and having electronic conductivity.
U.S. Pat. No. 4,057,480 discloses a process for operating a cell for the electrolysis of a molten charge, in particular aluminum oxide, with one or more anodes the working surfaces of which are of ceramic oxide material, and anode for carrying out the process. This patent also discloses that base materials for the anode may be SnO.sub.2, Fe.sub.2 O.sub.3, Fe.sub.3 O.sub.4, Cr.sub.2 O.sub.3, Co.sub.3 O.sub.4, NiO or ZnO, and that additions of one or more of the oxides of the following metals are found to be useful to improve the sinterability, the compactness and the conductivity of the SnO.sub.2 : Fe, Cu, Mn, Nb, Zn, Co, Cr, W, Sb, Cd, Zr, Ta, In, Ni, Ca, Ba, Bi.
U.S. Pat. No. 4,098,669 discloses sintered electrodes comprised of a self-sustaining matrix of sintered powders of yttrium oxide and at least one electroconductive agent, the electrode being provided over at least a portion of its surface with at least electrocatalyst useful for electrolysis reaction and bipolar electrodes with the matrix and electrolysis cells containing the electrodes.
U.S. Pat. No. 4,374,761 discloses an inert electrode composition suitable for use in the electrolytic production of metal from a metal compound dissolved in a molten salt. The electrode comprises a ceramic oxide composition and at least one metal powder dispersed through the ceramic oxide composition for purposes of increasing its conductivity, the metal powder selected from the group consisting of Ni, Cu, Co, Pt, Rh, In and Ir.
U.S. Pat. No. 4,454,015 discloses an inert electrode composition suitable for use as an inert electrode in the production of metals such as aluminum by the electrolytic reduction of metal oxide or metal salt dissolved in a molten salt bath. The composition comprises one or more metals or metal alloys and metal compounds which may include oxides of the metals comprising the alloy.
U.S. Pat. No. 4,455,211 discloses inert electrode compositions suitable for use in the production of metal by the electrolytic reduction of a metal compound dissolved in a molten salt. This patent discloses electrode compositions formed by reacting together two or more metal-containing reactants to provide an in situ displacement reaction.
The aforementioned patents are hereby incorporated herein by reference.
It is an object of the present invention to provide a non-consumable electrode which does not react with oxygen to form greenhouse gases, and which has chemical resistance to the molten salt.
The electrodes used in the MHD process for energy generation are exposed to high-temperature molten oxides generated from fossil-fuel-based reactants (i.e., from the ash obtained from coal). Electrodes used in electric arc furnaces for melting refractory oxides are also exposed to high-temperature oxide liquids. These oxides are quite corrosive at elevated temperatures and such corrosion can limit operational life. Potential electrode materials must be: i) resistant to corrosion by molten oxides, ii) electrically conductive, and iii) mechanically robust.
Another advantage of the present invention is that it may be used to provide electrodes used in the MHD process for energy generation.
Still another environmental problem is the large amount of depleted uranium that is currently stored in secure dump sites across the United States. For instance, over 700,000 metric tons of depleted uranium are stored in Ohio, Kentucky and Tennessee alone. Although depleted uranium has found some uses in applications such as in the armament industry, arising principally from its high-density, there remains a long felt need to provide additional safe, effective and economical uses for this material.
Accordingly, another object of the present invention is to provide uses for depleted uranium in order to provide a use for this nuclear waste material which is otherwise hazardous and costly to store.
Although described with respect to the Hall-Heroult process for producing aluminum from alumina-bearing ores, electric arc furnaces for the manufacture of steel and for the melting of refractory metals and ceramics, and the MHD process, it will be appreciated that similar advantages, may obtain in other applications of the present invention. Such advantages may become apparent to one of ordinary skill in the art in light of the present disclosure or through practice of the invention.